Site icon is a small icon associated with a website. It is displayed in the web browser’s url bar and it is also the bookmark icon.
To upload a site icon go to the Cogwheel Icon > Settings > General. Then click on Select Image next to Site Icon.
Your image can be png, jpg or ico, and the size of the site icon should be between 16x16 and 310x310 pixels. It is recommended to upload a big image.